VI. Resources for Teaching Science

Activity 19.5.1 – Classification of subatomic particles

Recent developments in sub-atomic theory have changed our perception of the nature of matter. Rather than just three fundamental particles (neutrons, electrons, protons), nuclear physicists now believe there are 12 or more, including 6 leptons and 6 quarks (table 19.12, download from sciencesourcebook.com). The electron, which is a type of lepton, is fundamental (composed of no other particles) while protons and neutrons are not fundamental because they are composed of quarks. The field of particle physics is changing rapidly, and new discoveries cause us to re-evaluate old classifications. Originally, subatomic particles were classified on the basis of their masses. The tiniest particles were known as leptons (lep- means "small" in Greek), while the heaviest particles were classified as baryons (bary- means "heavy" in Greek). Particles of intermediate size were termed mesons (mes- means "middle" in Greek).

(1) Is the original classification scheme of subatomic particles still applicable? Sort subatomic particles in the table in ascending order according to mass. Are baryons always the heaviest particles, and leptons the lightest? Is the original classification scheme still relevant? Explain.